Terephthalic acid is an organic compound with formula C6H4(COOH)2. This colourless solid is a commodity chemical, used as a precursor for polyester PET, used to make clothing and plastic bottles. World production in 1970 was around 1.75 million tonnes. By 2006, global purified terephthalic acid (PTA) demand exceeded 30 million tonnes. There is a smaller, but nevertheless significant, demand for terephthalic acid in the production of polybutylene terephthalate and several other engineering polymers. In the research laboratory, terephthalic acid is a component for the synthesis of metal-organic frameworks. Due to the wider applications of terephthalic acid, many methods for its manufacturing and purification have been developed in recent years.
Some of the prior art documents which disclose the processes for the synthesis of terephthalic acids are as follows.
U.S. Pat. No. 2,833,816 suggests a process for producing aromatic carboxylic acid such as terephthalic acid by oxidation of aromatic compounds such as para-xylene at 120 to 275° C. in the presence of acetic acid, metal catalyst and a bromine source. During the oxidation reaction, the intermediates that are formed are p-toulic acid and 4-carboxy-benzaldehyde (4-CBA). Para-toulic acid remains in soluble state in the solvent. However, 4-CBA co-crystallizes with the product due to its similar crystal structure. Terephthalic acid which contains 4-CBA is typically termed as crude terephthalic acid. The crude terephthalic acid (CTA) containing 4-CBA is undesirable in producing polyester as it acts as a chain terminating agent during polymerization. Hence CTA is required to be further purified by additional steps.
U.S. Pat. No. 7,094,925 mentions a process for the oxidation of an alkyl-aromatic compound which comprises admixing the aromatic compound with an oxidizing agent or sulfur compound in the presence of an ionic liquid and a nitrogen oxyacid species. Ionic liquid used in the process comprises an organic anion selected from the group consisting of trifluoroacetate, acetate, methanesulfonate, and combinations thereof or an anion based on sulfur, nitrogen, phosphorous, silicon, selenium, tellurium, arsenic, antimony, bismuth, or oxoanions of a metal. The process mentioned in U.S. Pat. No. 7,094,925 is carried out under Bronsted acidic conditions.
US2009/0326265 suggests the use of 1-ethyl-3-methylimidazolium bromide as a bromine source during oxidation. The use of molecular bromine typically releases free bromine which causes corrosion to equipment.
In U.S. Pat. No. 6,355,835, the use of methylethlyketone (MEK) as a promoter to replace the bromine source is suggested. However, the process in U.S. Pat. No. 6,355,835 needs a large quantity of catalyst. Another disadvantage of the process is that the organic promoter may undergo oxidation at the process operating conditions, further complicating purification steps.
U.S. Pat. No. 6,153,790 suggests the utilization of a catalyst system which is a combination of cobalt and zirconium species for the preparation of aromatic carboxylic acids.
US2010/0174111 suggests a process for providing crystalline terephthalic acid comprising: a) providing a composition comprising terephthalic acid and one or more ionic liquids; and b) combining the composition of step (a) with a non-solvent (water), thereby crystallizing terephthalic acid.
US20120004449 suggests a process for oxidizing an alkyl-aromatic compound which comprises forming a mixture comprising the alkyl-aromatic compound, a solvent, a bromine source, and a catalyst; and contacting the mixture with an oxidizing agent to produce a solid oxidation product comprising at least one of an aromatic aldehyde, an aromatic alcohol, an aromatic ketone, and an aromatic carboxylic acid; wherein the solvent comprises a carboxylic acid having from 1 to 7 carbon atoms and an ionic liquid selected from the group consisting of an imidazolium ionic liquid, a pyridinium ionic liquid, a phosphonium ionic liquid, a tetra alkyl ammonium ionic liquid, and combinations thereof.
US2012/0004450 suggests a process for producing terephthalic acid from para-xylene which comprises forming a mixture comprising the para-xylene, a solvent, a bromine source, and a catalyst; and oxidizing the para-xylene by contacting the mixture with an oxidizing agent to produce a solid oxidation product comprising terephthalic acid, 4-carboxybenzaldehyde, and para-toluic acid; wherein the solvent comprises a carboxylic acid having from 1 to 7 carbon atoms, a dialkyl imidazolium ionic liquid, and optionally water.
US2012/0004451 suggests a process for producing terephthalic acid from para-xylene which comprises forming a mixture comprising the para-xylene, a solvent, a bromine source, a catalyst and ammonium acetate; and oxidizing the para-xylene by contacting the mixture with an oxidizing agent to produce a solid oxidation product comprising terephthalic acid, 4-carboxybenzaldehyde, and para-toluic acid; wherein the solvent comprises a carboxylic acid having from 1 to 7 carbon atoms.
US2012/0004454 suggests a mixture for oxidizing an alkyl-aromatic compound comprising: the alkyl-aromatic compound, a solvent, a bromine source, and a catalyst; wherein the solvent comprises a carboxylic acid having from 1 to 7 carbon atoms and an ionic liquid selected from the group consisting of an imidazolium ionic liquid, a pyridinium ionic liquid, a phosphonium ionic liquid, a tetra alkyl ammonium ionic liquid, and combinations thereof.
The aforesaid documents focus on preparing terephthalic acid by oxidation of xylene in the presence of a catalyst, a solvent and a bromine source. The solvents utilized include acetic acid, water and ionic liquids.
These processes are not able to produce pure terephthalic acid. i.e. these processes are not able to reduce the formation of intermediates such as 4-carboxy-benzaldehyde. Therefore, these processes inherently require additional purification steps. Further, these processes utilize excess amount of bromine and metal catalyst.
Accordingly, it is desirable to develop a mixture which can undergo oxidation and produce terephthalic acid which is substantially free of intermediates such as 4-carboxy-benzaldehyde.
Objects
Some of the objects of the present disclosure which at least one embodiment herein satisfies are as follows:
It is an object of the present disclosure to provide a composition for the preparation of terephthalic acid.
It is another object of the present disclosure to provide a composition for the preparation of purified terephthalic acid, which is capable of reducing the formation of intermediates.
It is another object of the present disclosure to provide a simple, safe and cost effective process for the preparation of purified terephthalic acid.
Other objects and advantages of the present disclosure will be more apparent from the following description which is not intended to limit the scope of the present disclosure.